Task design has been viewed to be essential in the context of language assessment. This study investigated whether increasing task complexity affects learners' writing performance. It employs three writing tasks with different levels of complexity based on Robinson's Componential Framework. A cohort of 278 participants was selected using a simple random sampling technique, and a corpus of 603 compositions was used for data analysis. The results showed that learners demonstrated higher scores and higher levels of coherence in the compositions of more complex tasks. The findings of this study contribute to the growing field of cognitive research by exploring the effects of task complexity on learners' writing production. The results have implications for online automated writing assessment, writing test development, and the design of high-stakes language tests. The findings also provide recommendations on how tasks should be designed to develop learners' language production skills in writing instruction.